Particular embodiments generally relate to voltage controlled oscillators (VCOs).
Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
FIG. 1 depicts a conventional voltage controlled oscillator 100. An inductor/capacitor tank (LC tank) 101 is formed by a parallel or series connection of inductors 102a/102b and a capacitor 104. LC-tank 101 is coupled to an active circuit, which is represented as a cross-coupled transistor pair 106. As shown, cross-coupled transistor pair 106 is coupled in parallel to LC-tank 101 and includes a first transistor 108a (M1) and a second transistor 108b (M2).
In operation, for a resonant frequency, the impedance of LC-tank 101 becomes infinite and when energy is stored initially in LC tank 101, it circulates from voltage energy in capacitor 104 to current energy in inductors 102, and vice versa. This exchange of energy occurs at the resonant frequency, with the voltage and current being sinusoidal in quadrature phase with respect to each other and the ratio of the voltage and current amplitude being:V/I=√{square root over (LC)}. 
Reactive components, such as inductors 102 and capacitor 104, have losses in the real world implementation. The losses may be modeled as series or parallel resistances to LC-tank 101. The losses may dampen the oscillating signal generated by LC-tank 101. The active circuit may be used to compensate for the losses.
A negative resistance is synthesized by cross coupled transistor pair 106 and is explained by describing the currents sourced/sinked by cross-coupled transistor pair 106 to/away from LC-tank 101. The current sourced/sinked is biased by a current source (Ibias) 110. Current source 110 may be a current mirror in one example. When a voltage at a node Vp is at its positive peak value, the resistance of LC-tank 101 is taking away current from node Vp. To compensate for this, transistor 108a sinks less current from node Vp. When the voltage at node Vp is at its negative peak value, the resistance of LC-tank 101 is sourcing current into node Vp and transistor 108a is sinking current from node Vp. The dual behavior also happens at node Vn.
Cross-coupled transistor pair 106 is behaving as a negative resistance because cross-coupled transistor pair 106 is sourcing current from nodes Vp or Vn when the voltage is at a maximum at the nodes and sinking current from nodes Vp or Vn when the voltage is at a minimum at the nodes. The ratio between the voltage at nodes Vp or Vn to the current flowing out of nodes Vp or Vn is negative.
Cross-coupled transistor pair 106 synthesizes the negative resistance that sustains the oscillation at a desired frequency. However, cross-coupled transistor pair 106 introduces noise that contributes to the total phase noise of oscillator 100. This may cause the output of LC-tank 101, which is a sine wave, to include some noise. When the sine wave is input into a downstream component from oscillator 100, this noise may cause jitter on the output of the component. Also, the component contributes noise as well. The gradual slope of the sine wave increases jitter on the output of the component that is added by the component. This causes uncertainty in when the component may output the signal and may cause problems in operation of a chip.